Earth-boring drill bits are commonly used in drilling oil and gas wells or mineral mines. Typically, an earth-boring drill bit is mounted on the lower end of a drill string. As the drill string is rotated at the surface, the drill bit is rotated down in the borehole as well. With the weight of the drill string bearing down on the drill bit, the rotating drill bit engages an earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A rock bit, typically used in drilling oil and gas wells, generally includes one or more rotatable cones that perform their cutting function due to the rolling and sliding movement of the cones acting against the formation. The earth-disintegrating action of the rolling cone cutters is enhanced by a plurality of cutter elements. Cutter elements are generally inserts formed of a very hard material which are press-fitted into undersized apertures or sockets in the cone surface. Due to their toughness and high wear resistance, inserts formed of tungsten carbide in a cobalt binder are commonly used in rock-drilling and earth-cutting applications.
Breakage or wear of tungsten carbide inserts limits the lifetime of a drill bit. In a rock bit, inserts are subjected to high wear loads from contact with a borehole wall. Additionally, the inserts are exposed to high stress due to bending and impacting loads resulting from contact with a borehole bottom. The high wear load can also cause thermal fatigue which initiates surface cracks on the carbide inserts. These cracks are further propagated by a mechanical fatigue mechanism that is caused by the cyclical bending stress and/or impact loads applied to the inserts. The cracks may result in chipping, breakage, and failure of inserts.
Inserts that cut the corner of a borehole bottom generally are subject to the greatest amount of thermal fatigue. Thermal fatigue is caused by heat generation on the gage side of the insert. The heat results from a heavy frictional loading component that is produced as the insert engages the borehole wall and slides to the bottom-most crushing position. When the insert retracts from the bottom, it is quickly cooled by the surrounding circulating drilling fluid. This repetitive heating and cooling cycle can initiate cracking on the outer surface of the insert. These cracks then propagate through the body of the insert when the crest of the insert contacts the borehole bottom. The time required to progress from heat checking to chipping and eventually to a broken insert depends upon the formation type, rotation speed, and applied weight. Despite lower drilling speeds and air cooling, the problem of thermal fatigue is more severe in mining bits because greater weight is applied to the bit and the formations usually are harder. In petroleum bits, thermal fatigue also is a serious concern because of the faster bit rotation speed and cooling with drilling mud.
Cemented tungsten carbide generally refers to tungsten carbide ("WC") particles dispersed in a binder metal matrix (i.e., iron, nickel or cobalt). Tungsten carbide in a cobalt matrix is the most common form of cemented tungsten carbide. This type of tungsten carbide is further classified by grades based on the grain size of WC and the cobalt content. Existing tungsten carbide grades for inserts have been adjusted for desired wear resistance and toughness only. These carbide inserts frequently fail when high rotational speed and high weight are applied due to heat checking and thermal fatigue.
Because thermal fatigue plays a critical role in limiting the lifetime of a tungsten carbide insert and because existing carbide grades are not formulated to minimize thermal fatigue in inserts, there exists an unfulfilled need for inserts formed of an improved tungsten carbide composition which will minimize thermal fatigue while maintaining desired toughness and wear resistance.